PHOTONASTIC PHENOMENA AND DIURNAL SLEEP 635 



is no longer possible when the fundamental unity of effects 

 in the two cases has been perceived. 



But it is customary to make a further distinction between 

 these effects, on the ground that the movement of the radial 

 is determined by, while that of the anisotropic is independent 

 of, the directive action of light. In this matter again I shall 

 be able to show further that no such line of demarcation can 

 be drawn. It will then be seen that the differences between 

 the two classes of phenomena are only apparent. 



In the case of radial organs, we saw that the direct action 

 of moderate stimulus of light, inducing concavity of the 

 proximal, gave rise to positive heliotropic movement. The 

 same is true of the similar direct action of a moderate 

 intensity of light on, say, the pulvinus of Mimosa, where the 

 upper and lower halves of the organ both exhibit positive 

 response. With stronger light, again, the radial organ 

 develops an induced anisotropy, by which the proximal 

 side, owing to fatigue, becomes the relatively less excitable. 

 Under this condition, the proximal side of the radial organ 

 corresponds to the less excitable upper half of the pulvinus 

 of Mimosa, and the distal to the more excitable lower half. 

 This differentiation is seen to be continuous throughout the 

 three cases of the radial organ, the plagiotropic stem, and the 

 pulvinus, although in the first of these it is transient, lasting 

 only during the action of stimulus. In all these cases alike, 

 strong unilateral stimulus, acting on the less excitable 

 proximal or upper surface, becomes internally diffused, and 

 causes movement away from stimulus, or negative heliotropic 

 response, in a direction which is perpendicular to the plane 

 of separation of the two anisotropic halves. Thus there is 

 no difference between the negative heliotropic responses of 

 a radial and a pulvinated organ. 



The distinction between the two cases of the permanent 

 and temporary differentiation of the organ has been illustrated 

 by Sachs by a reference to the induction of polarity in steel 

 and in soft iron respectively. According to this analogy, 

 Marchantia behaves to intense light like steel to a magnet, 



